Approximately 50 junior and senior investigators will convene in a Workshop formally titled, "Assembling a Multi-Cellular Circadian Pacemaker: Seeking Principles of Neural Circuit Organization Across Invertebrate and Vertebrate Brains," with a working title of "Circadian System Circuits." This Workshop is a necessary response to several emerging themes in the field of circadian rhythm research. On the one hand, great strides have been made with respect to identification of the molecular clockworks which form the foundation of circadian timekeeping. On the other, stable circadian rhythmicity is recognized as a product of the clockworks being expressed through a system of interconnected neurons which govern the timing of efferent signals that regulate behavioral and physiological events. On the sensory side, with emphasis on photoreceptors, periodic environmental stimuli provide synchronizing input to the circadian system. Within the central circadian clock, individual components of the neuronal system are being detected and selectively evaluated. Many of these components are known anatomically and functionally in insects (most notably, Drosophila melanogaster). In contrast, there is a paucity of information about individual cell types in the mammalian clock, particularly with respect to their connections and the cell-cell signaling that presumably governs the coding of temporal information affecting timing of bodily events. The proposed Workshop is to be held in the Lorentz Center, Leiden University, The Netherlands. It will bring together internationally recognized senior faculty who study insects or mammals to mix with more junior faculty, post docs or students in an intimate forum for lively presentation and discussion of data, ideas and principles. The number of participants and the relatively narrow focus of the Workshop is expected to facilitate the flow of information. The goal of the Workshop is to create a sense of where the field is with respect to what is known about the multicellular circadian system. This will, both by default and design, demonstrate what kind of information is lacking and where research emphasis should be placed in the future. The Workshop will generate a product, in the form of a multi-author review that will summarize what is known about the circadian system. It will also provide a sense of future research goals that may provide useful guidance of investigators and oversight agencies. PUBLIC HEALTH RELEVANCE: The circadian rhythm system governs the timing of virtually all behavioral and physiological events. As such, it is an organizing principle in all life forms. An understanding of how the circadian system receives sensory information, generates an oscillatory output and conveys that output to target systems elsewhere in the brain and body is essential to the understanding of normal animal and human health. The proposed Workshop on Circuits of the Circadian System will gather established, new and still emerging scientists to examine the circadian system circuitry in order to determine the depth of knowledge about its organization and the extent to which scientific ignorance obstructs its understanding.